Purge compositions are used in polymer processing equipment, such as extrusion and injection molding equipment, as a cleaning agent. The purge compositions are used when the material which is being processed in the equipment is changed. For instance, in processing equipment, a change in the color of polymer requires removal of the polymer containing the first color before introduction of a polymer containing a second color.
A number of purge compositions are currently available to the polymer processing industry. These compositions generally consist of a carder medium, containing a dispersion of abrasive particles. The current carrier media consist of liquids, low melting, low viscosity solids (e.g. waxes and polymers) or higher viscosity polymers.
The lower viscosity carders generally have an advantage over higher viscosity materials because they tend to be more compatible and mix with a broader range of polymer materials. Low viscosity carriers generally are able to accept high levels of abrasive material. The low viscosity materials generally have low molecular weights and are believed to produce a greater change in entropy upon mixing with other materials. When the entropy change is greater, then there is a greater chance that mixing will occur. The ability for a carder to mix with and remove polymers from the processing equipment is important for purging operations.
In addition, when mixing liquids of different viscosity, it is believed that the lower viscosity liquids will tend to encapsulate the high viscosity materials. Accordingly, a low viscosity carrier system would be useful for purging higher viscosity polymers. To be effective, a purge composition must transfer mechanical energy from the moving rotor, e.g. a screw, to the stationary walls of the processing equipment. A problem of low viscosity carriers is that low viscosity liquids do not efficiently transfer this mechanical energy. Another problem with low viscosity carders is that they lubricate the screws and rotors of the mixing equipment. Additionally, if the low viscosity carder is liquid at room temperature, then special processing equipment is required to disperse the abrasive material in the low viscosity carrier.
High viscosity carder resins for purge compositions are able to transfer mechanical energy in internal mixers for better cleaning action. However, as the level of abrasive material is increased in high viscosity carder systems, the viscosity of the purge composition increases, making processing more difficult. High loading levels of particulate material also reduce the inherent strength of the mixture, making processing into pellets difficult. Also, they do not mix well with other polymers due to the low entropy change upon mixing.
Therefore, it is desirable to have abrasive compositions and purge compositions which efficiently transfer mechanical energy, with little or no screw slippage. It is desirable to have abrasive compositions and purge compositions which are effective at removing polymers from the processing equipment. It is also desirable to have abrasive compositions and purge compositions provide a means for rapid polymer change-over. It is desirable to have an abrasive composition which provides the benefits of low viscosity carders with the benefits of the high viscosity carder, while avoiding the deficiencies of these carriers.